1. Field of the Invention
The present invention relates generally to the copper foils used in making printed circuit boards. More particularly, it concerns attaching copper foils to sheets of other metals to provide physical support for the foil and to facilitate their lamination to insulating substrates, such as glass fiber-reinforced resin prepregs, e.g. epoxy resins, or polymer films such as polymides.
2. Description of Related Art
The use of metal sheets as carriers for copper foils is known in the art. Electrodeposition of copper on metal sheets has been proposed in order to obtain very thin layers of copper, which might otherwise not be feasible. Typical foils used in making printed circuit boards are about 9 to 35 .mu.m thick, but by electrodepositing on a supporting sheet of another metal it is possible to use thinner layers of copper. The copper is laminated to a substrate and then the supporting sheet is stripped away, leaving a thin layer of copper on the substrate from which circuit lines are made.
In U.S. Pat. No. 5,153,050 Johnston disclosed a method of supporting sheets of conventionally thick copper foil on heavier aluminum sheets and sealing the edges of the copper foil to the aluminum sheets with adhesive to prevent foreign particles from infiltrating between the two sheets and causing defects in the subsequently formed circuit lines. It has also been proposed to use a similar method using, instead of an aluminum sheet, another sheet of copper foil. For example, by applying adhesive to the edges of the sheets or mechanically joining the edges of the sheets.
While the method of Johnston has met with commercial success, it has certain disadvantages. Contamination of the copper foil and or the aluminum sheet by the adhesive may be a problem since all of the aluminum and part of the copper foil will be reclaimed after the copper foil has been laminated to the substrate. (The aluminum sheet is separated and a portion of the laminated board is trimmed away to remove the adhesive region). Applying adhesive and assembling the metal sheets is difficult and involves complex machinery to carry out on a commercial scale. Furthermore, copper and aluminum have different coefficients of thermal expansion so that during lamination, which is done under heat and pressure, distortion of the copper by the large expansion of aluminum may occur, introducing undesirable stresses in the copper clad laminate.
The present inventors have found an improved method of combining copper foils and aluminum or other metal sheets, which is simpler, less expensive, and avoids some of the problems discussed above. The method involves the aligning of continuous strips of copper foil and a second metal and the use of ultrasonic welding of the edges of the two metal strips. The bond is sufficient to permit handling of the joined sheets, but when separated no residual adhesive or extraneous metal is left to contaminate the copper and aluminum.